Planar lightwave circuits (PLC) are optical devices including planar optical waveguides formed on a silicon wafer substrate, wherein the waveguides are made from transmissive media which have a higher refractive index than the outlying cladding layers in order to guide light along the optical path. PLCs are designed to integrate multiple components and functionalities into a single optical chip, primarily for the telecommunications industry.
In designing any optical device for fabrication on a PLC, the design and fabrication of the optical waveguides is very important. The dimensions of the waveguides, and the separation of one waveguide from another, influence the stress on the waveguide, and hence the birefringence exhibited by the waveguide. The waveguide birefringence may give rise to undesirable polarization dependence properties such as polarization dependent loss (PDL) and polarization dependent wavelength of the optical device. For most component manufacturers and their customers it is important for the waveguide birefringence (B) through the device to be as low as possible, ideally zero, where B is commonly defined as the difference between the effective refractive indices experienced by TM and TE polarized light.
Polarization dependence of waveguide devices originates from stress induced birefringence in the waveguide layers which perturbs the desired index profile of the waveguide and leads to a difference in the effective modal index for the two polarizations.
Stress birefringence can lead to a difference in the effective index of the waveguide modes for the two polarizations, which is particularly detrimental in phase sensitive devices such as an arrayed waveguide grating (AWG) or a Mach-Zehnder Interferometer (MZI). For example, in an AWG, birefringence in the waveguides of the phase array leads to a splitting in the transmission spectrum of the device for the two polarizations, termed polarization dependent wavelength (PDW), resulting in an unacceptable polarization dependent loss if the PDW is large.
Polarization dependence in these devices is generally believed to be the result of differences of the thermal expansion properties between layers in the device structures causing stresses during the manufacturing process. Asymmetric stress in the waveguide cores results in birefringence.
U.S. Pat. No. 6,826,345 to Fan Zhong et al. issued Nov. 30, 2004 discloses a solution for birefringence in phase sensitive devices comprising a top cap layer of selected thickness on the top surface of the waveguide to reduce stress differences in the waveguide core. The method is said to reduce or substantially eliminate birefringence in an AWG device. For these devices, the polarization dependence is perturbed mainly by changes in the stress induced birefringence close to the center of the waveguide mode. The top cap does not cover the sidewalls of the waveguide core. Birefringence in the cladding layer is not considered.
Polarization dependence (such as PDL) is also of importance in devices that are not phase sensitive such as directional couplers, splitters and parabolic horns. For these devices, several factors can influence the polarization dependence of the design. However, it has been found that the stress, and hence birefringence, in the cladding is the more dominant factor in limiting the polarization sensitivity of the device than the waveguide core birefringence. This is particularly true for coupled mode devices such as the directional coupler or mode conversion horn. In both these examples, operation of the device is essentially dependent on co-propagation of fundamental and higher order mode(s), which are not as strongly confined to the waveguide core.
It is known in the prior art to dope the cladding to match its CTE to that of the substrate for the purposes of reducing waveguide birefringence (A. Kilian, J. Kirchhof, B. Kuhlow, G. Przyrembel and W. Wischmann ‘Birefringence free planar optical waveguide made by FHD through tailoring of the overcladding’ Journal of Lightwave Technology) but heavy doping affects the environmental stability of the device and the cladding to core stress is still present.
A further problem affecting the performance of planar waveguides is that during formation of the cladding and waveguiding layers gas pockets can be trapped under subsequent consolidated layers. Gas also becomes trapped between closely spaced waveguides such as directional couplers, Y splitters and similar structures. Such gas pockets disrupt the optical properties of the PLC layers. In an application by Kymata Limited, publication number GB 2,355,078 A, published Apr. 11, 2001, this problem is addressed by providing a composite cladding layer including a cladding interface between the waveguide core and an outer cladding portion. The cladding interface portion has a lower consolidation temperature than the outer cladding portion. Both cladding layers are consolidated together. Dopant ions are used to lower the softening temperature and maintain the refractive index. Doping is also suggested to match the thermal coefficient to that of the buffer layer. Birefringence in the cladding layer is not recognized as a source of PDL, however, and because the CTE is not matched between the core and cladding interface, the cladding interface does not separate the refractive index from the birefringence.
Accordingly, a method for reducing PDL in planar waveguide structures remains highly desirable.